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The Featured Molecules this month come from Donald C. Bowman’s article “A Colorful Look at the Chelate Effect”, the final Overhead Projector Demonstrations column edited by the late Doris Kolb. Included in the online collection are all eight isomeric forms of [Ni(en)3]2+, demonstrating the effects of ligand backbone conformation. The notation D and L refer to the overall chirality produced by the ligands, and d and l are the ligand conformations. Each of these structures was created independently, then minimized using the UFF force field, the Zindo/1 basis set, and finally the 6-31G basis set. In all cases, the energies of the enantiomeric pairs of isomers were identical, as expected. The range of energies for the various conformational isomers depends on the level of theory used, but the order of energies is always Dddl = Llld < Ddll = Lldd < Dddd = Llll < Dlll = Lddd. Frequency calculations for all of these structures showed no imaginary frequencies, verifying that the structures are at least local minima. Calculations starting with quite different initial geometries all converged to the same final geometry, indicating a high probability that the structures are global minima as well.
Also included in the collection are all of the other molecules in Table 1 of the Bowman paper, and the hexaaquo complexes of Ni(II) and Cu(II). Proposing structures for Cu(II) complexes in solution is always problematic. All of the structures of Cu(II) complexes in Table 1 should be based on an octahedral coordination geometry, with the assumption that water molecules occupy two trans sites in the complex. As a result of Jahn–Teller distortions, the complexes are often represented with just four ligands in a (possibly distorted) square–planar arrangement. The structures of the Cu(II) complexes in this month’s collection were calculated using the ZIND/1 basis set, with two molecules placed along the z axis at a distance 30% greater than that found from a static calculation of Cu(H2O)62+; the water molecules were then removed after the minimization was complete.
Fully manipulable (Chime and Jmol) versions of these and other molecules are available at the JCE Digital Library Web site.
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